Semiconductor sensor and method for manufacturing the same

ABSTRACT

A semiconductor sensor includes: a semiconductor substrate; a plurality of piezoelectric thin films layered on the semiconductor substrate, the plurality of piezoelectric thin films including at least a pair of the piezoelectric thin films layered above and below; a pair of electrodes that are formed at an interface of at least the pair of the piezoelectric thin films layered above and below and excite surface acoustic waves; a thin film directly under a lowest-layer piezoelectric film of the piezoelectric thin films; a metal thin film that is formed at an interface of the lowest-layer piezoelectric thin film and the thin film, and facilitate a growth of a ridge-and-valley portion on a surface of an uppermost-layer piezoelectric thin film of the piezoelectric thin films; and a sensitive film for molecular adsorption formed on at least the ridge-and-valley portion on the uppermost-layer piezoelectric thin film.

This is a divisional application of U.S. Ser. No. 12/336,846 filed Dec.17, 2008 which claims priority to Japanese Patent Application No.2008-012925 filed Jan. 23, 2008. The entire disclosures of each of theabove applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a semiconductor sensor and a method formanufacturing the semiconductor sensor.

2. Related Art

Recently, chemical sensors, odor sensors, gas sensors, and the like thatdetect chemical substances in the air have been developed in response toincreasing awareness of environmental issues. In the sensors, asensitive film on which chemical substances adsorb is formed on apiezoelectric element, such as a quartz crystal resonator and a surfaceacoustic wave element, and the mass change of the sensitive filmcorresponds to the oscillation frequency change of the piezoelectricelement. As a result, the chemical substances can be detected by usingthe mass change.

For example, JP-A-2007-147556 discloses a method for manufacturing athin film (sensitive film) and a chemical sensor using the thin filmmanufactured by the method. In order to improve adsorption sensitivity(i.e., the sensitivity of the sensor), the method includes: a step formixing a sensitive film material and fine particles; a step for forminga thin film with a mixture of the sensitive film material and the fineparticles; a step for drying the thin film; and a step for removing thefine particles exposed at the thin film surface after being dried,thereby increasing the adsorption area of the thin film.

The related art described above has a problem in that such particularsteps are additionally required in order to improve the adsorptionsensitivity of the sensitive film, increasing the manufacturing costsand the price of the chemical sensor.

SUMMARY

An advantage of the invention is to provide a semiconductor sensorhaving a high sensitivity with a low price, and a method formanufacturing the semiconductor sensor.

According to a first aspect of the invention, a semiconductor sensorincludes: a semiconductor substrate; a plurality of piezoelectric thinfilms layered on the semiconductor substrate, the plurality ofpiezoelectric thin films including at least a pair of the piezoelectricthin films layered above and below; a pair of electrodes that are formedat an interface of at least the pair of the piezoelectric thin films andexcite surface acoustic waves; a thin film directly under a lowest-layerpiezoelectric film of the piezoelectric thin films; a metal thin filmthat is formed at an interface of the lowest-layer piezoelectric thinfilm and the thin film, and facilitate a growth of a ridge-and-valleyportion on a surface of an uppermost-layer piezoelectric thin film ofthe piezoelectric thin films; and a sensitive film for molecularadsorption formed on at least the ridge-and-valley portion on theuppermost-layer piezoelectric thin film.

In the semiconductor sensor, the metal thin film formed at the interfaceof the lowest-layer piezoelectric thin film and the thin film directlyunder the lowest-layer piezoelectric thin film can produce theridge-and-valley portion on the surface of the uppermost-layerpiezoelectric thin film. That is, the surface area of the sensitive filmfor molecular adsorption formed on the surface of the uppermost-layerpiezoelectric thin film (i.e., molecular adsorption area) can beenlarged, providing a semiconductor sensor having a high sensitivity. Inaddition, the metal thin film can be formed by using conventionalsemiconductor manufacturing processes without employing a particularprocess for enlarging the surface area of the sensitive film asnecessary in related art, e.g., JP-A-2007-147556. Thus, themanufacturing costs can be reduced. Consequently, a semiconductor sensorhaving a high sensitivity can be provided with low price.

In the sensor, the metal thin film is preferably a metal facilitating acrystal growth of a wurtzite structure of the piezoelectric thin filmsin a c-axis direction.

The metal can facilitate the growth of the ridge-and-valley portion onthe surface of the uppermost-layer piezoelectric thin film.

In addition, it is preferable that any one of Pt, Au, Al, Ag, Cu, Mo,Cr, Nb, W, Ni, Fe, Ti, Co, Zn, and Zr be used for the metal thin film.

The sensor preferably includes an oscillation circuit. The circuitpreferably includes: an inverter circuit layered between thelowest-layer piezoelectric thin film and the semiconductor substrate;and a surface acoustic wave element having the metal thin film, the pairof the piezoelectric thin films, the pair of the electrodes, and thesensitive film. In the sensor, the inverter circuit and the acousticwave element are preferably electrically coupled.

The oscillation frequency of such inverter type oscillation circuitdepends on the frequency characteristics of the surface acoustic waveelement. The frequency characteristics of the surface acoustic waveelement vary depending on molecular adsorption amount of the sensitivefilm since the sensitive film on which molecules adsorb is provided tothe surface acoustic wave element. Accordingly, the oscillationfrequency of the oscillation circuit varies. Measuring the oscillationfrequency change corresponding to the molecular adsorption amount withthe frequency counter and the like outside the sensor allows detectingchemical substances in the air with a high sensitivity.

In the sensor, it is preferable that the inverter circuit include a CMOScircuit.

The inverter circuit including the CMOS circuit can provide asemiconductor sensor having a low power consumption and high responsespeed.

In the sensor, it is preferable that the oscillation circuit include aplurality of the oscillation circuits. The surface acoustic wave elementincluded in at least one of the plurality of oscillation circuits ispreferably used as a reference element having no sensitive film.

The oscillation frequency of the oscillation circuit including thesurface acoustic wave element having no sensitive film, i.e., thereference element, is used as a reference frequency. Comparing theoscillation frequencies of the other oscillation circuits with thereference frequency allows more easily detecting the frequency varied byadsorption of gas molecules. As a result, a semiconductor sensor havinga higher sensitivity can be provided.

According to a second aspect of the invention, a method formanufacturing a semiconductor sensor includes: (a) layering a pluralityof piezoelectric thin films on a semiconductor substrate, the pluralityof piezoelectric thin films including at least a pair of thepiezoelectric thin films layered above and below; (b) forming a pair ofelectrodes at an interface of the pair of the piezoelectric thin films,the electrodes exciting surface acoustic waves; (c) forming a metal thinfilm at an interface of a lowest-layer piezoelectric thin film of thepiezoelectric thin films and a thin film directly under the lowest-layerpiezoelectric film, the metal thin film facilitating a growth of aridge-and-valley portion on a surface of an uppermost-layerpiezoelectric thin film of the piezoelectric thin films; and (d) forminga sensitive film for molecular adsorption at least on theridge-and-valley portion on the uppermost-layer piezoelectric thin film.

According to the method, the surface area of the sensitive film formolecular adsorption formed on the surface of the uppermost-layerpiezoelectric thin film (i.e., molecular adsorption area) can beenlarged, thereby allowing manufacturing a semiconductor sensor having ahigh sensitivity. In addition, the metal thin film can be formed byusing conventional semiconductor manufacturing processes withoutemploying a particular process for enlarging the surface area of thesensitive film as necessary in related art, e.g., JP-A-2007-147556.Thus, the manufacturing costs can be reduced. Consequently, asemiconductor sensor having a high sensitivity can be manufactured withlow costs.

In the method, the metal thin film is preferably a metal facilitating acrystal growth of a wurtzite structure of the piezoelectric thin filmsin a c-axis direction.

In addition, it is preferable that any one of Pt, Au, Al, Ag, Cu, Mo,Cr, Nb, W, Ni, Fe, Ti, Co, Zn, and Zr be used for the metal thin film.Further, in step (c), it is preferable that the metal thin film beformed by using Pt with a thickness of 1000 angstrom or more.

The metal thin film formed with Pt with a thickness of 1000 angstrom ormore can achieve the full-width at half-maximum of the X-ray diffractionpattern, which is an index of the crystal property, of 2 degrees orless.

In the method, it is preferable in step (a) that the lowest-layerpiezoelectric thin film be formed with AlN by using a reactivesputtering method by sputtering a pure Al target in an atmospherecontaining Ar and N₂ with a thickness of 8000 angstrom to 15000 angstromunder a film forming condition in which film forming pressure is withina range from 0.05 Pa to 2.0 Pa, a semiconductor substrate temperature iswithin a range of 150 degree centigrade to 400 degrees centigrade, and aflow rate ratio of Ar to N₂ is within a range of 0.0 to 1.0. The flowrate ratio of Ar to N₂ is 1.0 means that the gas flow rates areexpressed as Ar: N₂=1:1.

With this film forming condition, the lowest-layer piezoelectric thinfilm, which is formed on the metal thin film, can grow as a columnarcrystal orientated and aligned in the c-axis direction of the wurtzitestructure so as to produce the ridge-and-valley portion corresponding tothe grown crystal grain size on the uppermost surface of thelowest-layer piezoelectric thin film. In addition, since thepiezoelectric thin film formed on the lowest-layer piezoelectric thinfilm grows following the surface morphology of the lowest-layerpiezoelectric thin film, the crystal grain size grows larger, therebyallowing more enlarging the ridge-and-valley portion produced on thesurface of the uppermost-layer piezoelectric thin film, i.e., moreenlarging the surface area of the sensitive film.

The method preferably further includes: (e) forming an inverter circuitbetween the lowest-layer piezoelectric thin film and the semiconductorsubstrate; and (f) forming a wiring line electrically coupling a surfaceacoustic wave element including the metal thin film, the pair of thepiezoelectric thin films, the pair of the electrodes, and the sensitivefilm, and the inverter circuit so as to structure an oscillationcircuit.

The oscillation frequency of such inverter type oscillation circuitdepends on the frequency characteristics of the surface acoustic waveelement. The frequency characteristics of the surface acoustic waveelement vary depending on molecular adsorption amount of the sensitivefilm since the sensitive film on which molecules adsorb is provided tothe surface acoustic wave element. Accordingly, the oscillationfrequency of the oscillation circuit varies. Measuring the oscillationfrequency change corresponding to the molecular adsorption amount withthe frequency counter and the like outside the sensor allows detectingchemical substances in the air with a high sensitivity.

In the method, it is preferable in step (e) that a CMOS circuit beformed as the inverter circuit.

The inverter circuit including the CMOS circuit can manufacture asemiconductor sensor having a low power consumption and high responsespeed.

In the method, it is preferable that the oscillation circuit include aplurality of the oscillation circuits. In this case, in step (d), it ispreferable that the sensitive film be not formed in the surface acousticwave element included in at least one of the plurality of oscillationcircuits.

The oscillation frequency of the oscillation circuit including thesurface acoustic wave element having no sensitive film, i.e., thereference element, is used as a reference frequency. Comparing theoscillation frequencies of the other oscillation circuits with thereference frequency allows more easily detecting the frequency varied byadsorption of gas molecules. As a result, a semiconductor sensor havinga higher sensitivity can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1A is a sectional view of a semiconductor sensor according to anembodiment of the invention.

FIG. 1B is a detailed view of a portion of the semiconductor sensor ofthe embodiment.

FIG. 2A is a top view of the semiconductor sensor of the embodiment.

FIG. 2B is an equivalent circuit diagram of the semiconductor sensor ofthe embodiment.

FIGS. 3A to 3D are explanatory views of a method for manufacturing thesemiconductor sensor of the embodiment.

FIGS. 4A to 4D are explanatory views of the method for manufacturing thesemiconductor sensor of the embodiment.

FIGS. 5A to 5C are explanatory views of the method for manufacturing thesemiconductor sensor of the embodiment.

FIGS. 6A to 6C are explanatory views of the method for manufacturing thesemiconductor sensor of the embodiment.

FIGS. 7A to 7C are explanatory views of the method for manufacturing thesemiconductor sensor of the embodiment.

FIGS. 8A and 8B are explanatory views of the method for manufacturingthe semiconductor sensor of the embodiment.

FIGS. 9A and 9B are explanatory views illustrating modifications of thesemiconductor sensor of the embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention are described with reference to theaccompanying drawings.

First Embodiment Semiconductor Sensor

FIG. 1A is a sectional view illustrating a semiconductor sensor SSaccording to a first embodiment of the invention. FIG. 1B is a detailedview of a portion of the semiconductor sensor SS shown in FIG. 1A. FIG.2A is a top view of the semiconductor sensor SS. FIG. 2B is anequivalent circuit diagram of the semiconductor sensor SS.

As shown in FIG. 1A, the semiconductor sensor SS of the first embodimentincludes a semiconductor substrate 10, a local oxidization of silicon(LOCOS) film 20, a complementary metal oxide semiconductor (CMOS)circuit 30, a first interlayer film 40, a CMOS output electrode 50, asecond interlayer film 60, a metal thin film 70, a first piezoelectricthin film 80, a first electrode 90 a, a second electrode 90 b, a secondpiezoelectric thin film 100, and a sensitive film 110. The metal thinfilm 70, the first piezoelectric thin film 80, the first electrode 90 a,the second electrode 90 b, the second piezoelectric thin film 100 andthe sensitive film 110 are included in a surface acoustic wave element120.

The semiconductor substrate 10 is a silicon (Si) substrate. In theembodiment, a P-type Si substrate is exemplarily used. The LOCOS film 20is a silicon oxide film formed on the semiconductor substrate 10 by aLOCOS separation method to electrically separate a CMOS circuit formingregion and a surface acoustic wave element forming region.

The CMOS circuit 30 includes a separation insulation layer 31, a Pchannel type MOS transistor (hereinafter, referred to as a P-MOStransistor) 30 a and an N channel type MOS transistor (hereinafter,referred to as an N-MOS transistor) 30 b. The P-MOS transistor 30 a andthe N-MOS transistor 30 b are electrically separated by the separationinsulation layer 31. The P-MOS transistor 30 a includes an N-type well32 a, a gate insulation film 33 a formed on the N-type well 32 a, a gateelectrode 34 a formed on the gate insulation film 33 a, a source region35 a, and a drain region 36 a. The N-type well 32 a is formed by dopingN-type impurity ions into the semiconductor substrate 10. The sourceregion 35 a and the drain region 36 a are formed by doping P-typeimpurity ions into the N-type well 32 a. The N-MOS transistor 30 bincludes a gate insulation film 33 b formed on the semiconductorsubstrate 10, a gate electrode 34 b formed on the gate insulation film33 b, a source region 35 b, and a drain region 36 b. The source region35 b and the drain region 36 b are formed by doping N-type impurity ionsinto the semiconductor substrate 10.

The first interlayer film 40 is an insulation film formed on the LOCOSfilm 20 and the CMOS circuit 30 by a chemical vapor deposition (CVD)method or a spin coating method. Examples of the insulation film includea silicon dioxide (SiO₂) film, a tetra-ethoxy-silane (TEOS) film, aphosphor silicate glass (PSG) film, a boron phosphor silicate glass(BPSG) film, and combinations of them. In the embodiment, the firstinterlayer film 40 is composed of a SiO₂ film and a TEOS film depositedon the SiO₂ film. The first interlayer film 40 has a contact hole 40 acoupling the CMOS circuit 30 and the surface acoustic wave element 120.The CMOS output electrode 50 serves as an output side electrode of theCMOS circuit 30 and formed by patterning, with a photolithographymethod, a metal layer (e.g. Al—Cu) deposited by a sputtering method onthe contact hole 40 a and the first interlayer film 40. That is, theCMOS output electrode 50 electrically connects the drain region 36 a ofthe P-MOS transistor 30 a and the drain region 36 b of the N-MOStransistor 30 b in the CMOS circuit 30. The second interlayer film 60 isan insulation film formed on the first interlayer film 40 by a CVDmethod. The same material used for the first interlayer film 40 can beused for the second interlayer film 60. In the embodiment, a TEOS filmis used as the second interlayer film 60.

The metal thin film 70 is made of a metal having a function tofacilitate a crystal growth of a wurtzite structure of the firstpiezoelectric thin film 80 in a c-axis direction, which will bedescribed later. The metal thin film 70 is formed in the surfaceacoustic wave element forming region on the second interlayer film 60 bybeing patterned with a photolithography method. Examples of the metalused for the metal thin film 70 include platinum (Pt), gold (Au),aluminum (Al), silver (Ag), copper (Cu), molybdenum (Mo), chromium (Cr),niobium (Nb), tungsten (W), nickel (Ni), iron (Fe), titanium (Ti),cobalt (Co), zinc (Zn), zirconium (Zr), and combinations of them. In theembodiment, a combination of Ti and Pt is used.

The first piezoelectric thin film 80 is made of aluminum nitride (AlN)having piezoelectricity, and is formed by a reactive sputtering methodon the second interlayer film 60 and the metal thin film 70. Thepiezoelectric material used for the piezoelectric thin film 80 mayinclude metal oxide type piezoelectric materials such as zinc oxide(ZnO), lead zirconium titanate (PZT), lithium niobate (LiNO₃), andlithium tantalite (LiTaO₃). The first piezoelectric thin film 80 and thesecond interlayer film 60 have a via hole 80 a electrically coupling theCMOS output electrode 50 and the first electrode 90 a, i.e., wiring theCMOS circuit 30 and the surface acoustic wave element 120.

The first electrode 90 a and the second electrode 90 b are a pair ofelectrodes for exciting surface acoustic waves and formed by patterning,with a photolithography method, a metal layer (e.g., Al) deposited onthe via hole 80 a and the first piezoelectric thin film 80 by asputtering method. As shown in FIG. 2A, the first electrode 90 a and thesecond electrode 90 b constitute an interdigital transducer having adistance L between the electrodes as a constant pitch. The firstelectrode 90 a is electrically coupled to the CMOS output electrode 50while the second electrode 90 b is electrically coupled to an inputelectrode of the CMOS circuit 30. The input electrode of the CMOScircuit 30 is electrically coupled to the gate electrode 34 a of theP-MOS transistor 30 a and the gate electrode 34 b of the N-MOStransistor 30 b. In FIG. 1A, it is not shown.

The second piezoelectric thin film 100 is made of AlN havingpiezoelectricity as used for the first piezoelectric thin film 80, andformed by a reactive sputtering method on the first piezoelectric thinfilm 80, the first electrode 90 a, and the second electrode 90 b. Thesensitive film 110 is a thin film on which molecules of chemicalsubstances in the air adsorb, and formed in the surface acoustic waveelement forming region on the second piezoelectric thin film 100 bybeing patterned with a photolithography method. Examples of thesensitive film 110 include a synthetic polymer film mainly containingpolyester, polyamide, or the like, a natural polymer film such as lipid,and a silica series inorganic compound such as a silicone series polymerfilm.

The surface acoustic wave element 120 includes the metal thin film 70,the first piezoelectric thin film 80, the first electrode 90 a, thesecond electrode 90 b, the second piezoelectric thin film 100, and thesensitive film 110. Here, the metal thin film 70 has a function tofacilitate the crystal growth of a wurtzite structure of the firstpiezoelectric thin film 80 in the c-axis direction. Thus, the firstpiezoelectric thin film 80, which is formed on the metal thin film 70,grows as a columnar crystal orientated and aligned in the c-axisdirection of the wurtzite structure when the first piezoelectric thinfilm 80 is formed under predetermined film forming conditions. As aresult, a ridge-and-valley portion corresponding to the grown crystalgrain size is formed on the uppermost surface of the first piezoelectricthin film 80. In addition, the second piezoelectric thin film 100 growsfollowing the surface morphology of the first piezoelectric thin film80. Thus, the second piezoelectric thin film 100 grows to have a crystalgrain size larger than that of the first piezoelectric thin film 80. Asa result, the ridge-and-valley portion larger than that of the firstpiezoelectric thin film 80 is formed on the uppermost surface of thesecond piezoelectric thin film 100.

Since the sensitive film 110 is formed on the second piezoelectric thinfilm 100 on which the ridge-and-valley portion is formed, the sensitivefilm 110 has a similar ridge-and-valley portion. Thus, the surface areaof the sensitive film 110 enlarges. That is, an area on which gasmolecules adsorb can be enlarged, providing the semiconductor sensor SShaving a high sensitivity. In addition, the metal thin film 70 can beformed by using conventional semiconductor manufacturing processeswithout employing a particular process for enlarging the surface area ofthe sensitive film 110 as necessary in related art, e.g.,JP-A-2007-147556. Thus, the manufacturing costs can be reduced.Consequently, the semiconductor sensor SS having a high sensitivity canbe provided with low price. A method for manufacturing the semiconductorsensor SS will be described in detail later.

The operation principle of the semiconductor sensor SS is described withreference to the equivalent circuit diagram shown in FIG. 2B. As shownin FIG. 2B, the semiconductor sensor SS includes the P-MOS transistor 30a and the N-MOS transistor 30 b that are included in the CMOS circuit30, the surface acoustic wave element 120, a feedback resistor 200, aninput side capacitor 210 and an output side capacitor 220. The feedbackresistor 200, the input side capacitor 210 and the output side capacitor220 are not shown in FIG. 1A.

The output terminal of the CMOS circuit 30, i.e., the CMOS outputelectrode 50, is connected to one electrode of the surface acoustic waveelement 120, i.e., the first electrode 90 a, one terminal of thefeedback resistor 200, and one terminal of the output side capacitor220, and also connected to a frequency counter 300 provided outside thesemiconductor sensor SS through an output terminal P_(OUT). The inputterminal of the CMOS circuit 30, i.e., the gate electrode 34 a of theP-MOS transistor 30 a and the gate electrode 34 b of the N-MOStransistor 30 b, is connected to the other electrode of the surfaceacoustic wave element 120, i.e., the second electrode 90 b, the otherterminal of the feedback resistor 200, and one terminal of the inputside capacitor 210. A source electrode (not shown in FIG. 1A) of theP-MOS transistor 30 a is connected to an external power source VDDthrough a power source terminal P_(VDD). A source electrode (not shownin FIG. 1A) of the N-MOS transistor 30 b is connected to the groundthrough a ground terminal P_(GND). The other terminals of the input sidecapacitor 210 and the output side capacitor 220 are connected to theground through the ground terminal P_(GND).

That is, the semiconductor sensor SS includes an inverter typeoscillation circuit in which the CMOS circuit 30 is used as an inverterand the surface acoustic wave element 120 is used as an oscillationelement. The oscillation frequency of such inverter type oscillationcircuit depends on the frequency characteristics of the surface acousticwave element 120. The frequency characteristics of the surface acousticwave element 120 vary depending on molecular adsorption amount of thesensitive film 110 since the sensitive film 110 on which moleculesadsorb is provided to the surface acoustic wave element 120. As aresult, the oscillation frequency of the oscillation circuit varies.Measuring the oscillation frequency change corresponding to themolecular adsorption amount with the frequency counter 300 outside thesensor allows chemical substances in the air to be detected with a highsensitivity. Here, the distance L between the first electrode 90 a andthe second electrode 90 b that are included in the interdigitaltransducer is adequately set taking consideration into the wavelength ofthe oscillation frequency.

Second Embodiment Method for Manufacturing a Semiconductor Sensor

A method for manufacturing the semiconductor sensor SS according to asecond embodiment of the invention is described with reference to FIGS.3A to 8B.

First, the CMOS circuit forming region and the surface acoustic waveelement forming region are electrically separated on the semiconductorsubstrate 10 by using a LOCOS separation method. As shown in FIG. 3A, asilicon nitride film (SiNx) 11 is deposited on the semiconductorsubstrate 10 (P-type Si substrate) by using a CVD method. Then, as shownin FIG. 3B, the silicon nitride film 11 in the CMOS circuit formingregion is left while the silicon nitride film 11 in the surface acousticwave element forming region is removed (etched) by patterning thesilicon nitride film 11 with a photolithography method. Next, as shownin FIG. 3C, a silicon oxide film (the LOCOS film 20) is formed byoxidizing, at a high temperature, a region from which the siliconnitride film 11 has been removed. Then, as shown in FIG. 3D, the siliconnitride film 11 left in the CMOS circuit forming region is removed withheated phosphoric acid after the LOCOS film 20 is formed.

Through the above steps, the CMOS circuit forming region and the surfaceacoustic wave element forming region are electrically separated on thesemiconductor substrate 10.

Subsequently, as shown in FIG. 4A, the separation insulation layer 31 isformed to electrically separate the P-MOS transistor 30 a and the N-MOStransistor 30 b in the CMOS circuit forming region on the semiconductorsubstrate 10, and then N-type impurity ions are doped into thesemiconductor substrate 10 by an ion implantation method so as to fromthe N-type well 32 a. The separation insulation layer 31 may be formedat the same time in forming the LOCOS film 20, or may be formed by usinga shallow trench isolation (STI) method in a different step from one forforming the LOCOS film 20. Alternatively, the N-type impurity ions maybe doped before the LOCOS film 20 is formed. Next, as shown in FIG. 4B,the gate insulation film 33 is formed on the semiconductor substrate 10and the LOCOS film 20 by being thermally oxidized by using a CVD method,and then a poly-Si layer 34 serving as the gate electrode of the CMOScircuit 30 is deposited on the gate insulation film 33 by using a CVDmethod.

Next, as shown in FIG. 4C, the gate insulation film 33 and the poly-Silayer 34 are removed from a region excluding the gate electrodes of theP-MOS transistor 30 a and the N-MOS transistor 30 b by being patternedwith a photolithography method so as to form the gate insulation film 33a and the gate electrode 34 a of the P-MOS transistor 30 a and the gateinsulation film 33 b and the gate electrode 34 b of the N-MOS transistor30 b. Then, as shown in FIG. 4D, P-type impurity ions are doped into theN-type well 32 a by using an ion implantation method so as to form thesource region 35 a and the drain region 36 a of the P-MOS transistor 30a. Likewise, by using an ion implantation method, N-type impurity ionsare doped into the semiconductor substrate 10 so as to form the sourceregion 35 b and the drain region 36 b of the N-MOS transistor 30 b.After being doped, the impurity ions are heat treated so as to beactivated.

Through the above steps, the CMOS circuit 30 (P-MOS transistor 30 a andthe N-MOS transistor 30 b) is formed in the CMOS circuit forming regionon the semiconductor substrate 10.

Subsequently, insulation sidewalls for gate electrodes 34 a and 34 b areformed if required. Then, as shown in FIG. 5A (sidewalls are not shown),the first interlayer film 40 is formed on the semiconductor substrate 10(including the CMOS circuit 30) and the LOCOS film 20. In theembodiment, the first interlayer film 40 is formed as follows. SiO₂ filmis formed with a thickness of about 1000 angstrom by using a CVD methodas a low temperature oxide (LTO) film or a high temperature oxide (HTO)film. Then, a TEOS film is deposited on the SiO₂ film with a thicknessof about 8000 angstrom by using a CVD method. Examples of the insulationfilm used for the first interlayer film 40 may include a phosphorsilicate glass (PSG) film, a boron phosphor silicate glass (BPSG) film,and combinations of them in addition to the silicon dioxide (SiO₂) filmand the tetra-ethoxy-silane (TEOS) film described above. After the firstinterlayer film 40 is formed, the contact hole 40 a is formed by etchingso as to couple the CMOS circuit 30 to the surface acoustic wave element120. In this step, it is necessary that part of the drain region 36 a ofthe P-MOS transistor 30 a and part of the drain region 36 b of the N-MOStransistor 30 b are included in the opening region of the contact hole40 a.

Then, as shown in FIG. 5B, a metal layer (e.g., Al—Cu) deposited on thecontact hole 40 a and the first interlayer film 40 by a sputteringmethod are patterned by a photolithography method and etched so as toform the CMOS output electrode 50. As a result, the drain region 36 a ofthe P-MOS transistor 30 a and the drain region 36 b of the N-MOStransistor 30 b in the CMOS circuit 30 are electrically coupled. Next,as shown in FIG. 5C, a TEOS film is exemplarily formed on the firstinterlayer film 40 and the CMOS output electrode 50 as the secondinterlayer film 60 by using a CVD method, and then the surface of thesecond interlayer film 60 is planarized by using a chemical mechanicalpolishing (CMP) method. When a second metal wiring layer (e.g., gatewiring lines connected to the gate electrodes 34 a and 34 b, and sourcewiring lines connected to the source regions 35 a and 35 b) is requiredin the CMOS circuit 30, a third interlayer film (not shown) is formedand planarized.

Next, as shown in FIG. 6A, the metal thin film (Pt) 70 is deposited onthe second interlayer film 60 by using a CVD method. Then, as shown inFIG. 6B, the metal thin film 70 is left only in the surface acousticwave element forming region on the second interlayer film 60 by beingpatterned by using a photolithography method and etched. As describedabove, the metal thin film 70 is made of a metal having a function tofacilitate the crystal growth of the wurtzite structure of the firstpiezoelectric thin film 80, which is formed in the subsequent step, inthe c-axis direction. Thus, the crystal property of the metal thin film70 significantly influences the crystal property of the firstpiezoelectric thin film 80. From this point of view, the metal thin film70 preferably has a thicker thickness. In the embodiment, the full-widthat half-maximum of the X-ray diffraction pattern, which is an index ofthe crystal property, can be 2 degrees or less when the thickness of themetal thin film 70 made of Pt is 1000 angstrom or more. As metal for themetal thin film 70, one of the following metals having a lower value ofthe full-width at half-maximum may be used in addition to Pt. The metalsare Au, Al, Ag, Cu, Mo, Cr, Nb, W, Ni, Fe, Ti, Co, Zn, and Zr.

In order to improve adhesiveness between the metal thin film 70 and thesecond interlayer film 60 under the film 70, a Ti film may be formedwith a thickness of about 100 angstrom to about 1000 angstromtherebetween. Examples of such material for a thin film improving theadhesiveness may include silicide metals of Co, Ni, W, and Mo, inaddition to Ti.

Then, as shown in FIG. 6C, the first piezoelectric thin film 80 (AlN) isformed on the second interlayer film 60 and metal thin film 70 with athickness of from about 8000 angstrom to about 15000 angstrom. In theembodiment, the first piezoelectric thin film 80 was formed by using areactive sputtering method in which a pure Al target is sputtered in anatmosphere containing Ar and N₂. In order to grow the crystal grain ofthe AlN film to a larger size, the conditions were set and regulatedwithin the following ranges: the film forming pressure was from 0.05 Pato 2.0 Pa; the substrate temperature was from 150 degrees centigrade to400 degrees centigrade; and the flow rate ratio of Ar/N₂ was from 0.0 to1.0. As the result, it was confirmed that the first piezoelectric thinfilm 80 on the metal thin film 70 was grown to a columnar crystaloriented and aligned in the c-axis direction of the wurtzite structurewith a grain size of 80 nm to 200 nm. In addition, a ridge-and-valleyportion corresponding to the grown crystal grain size formed on theuppermost surface of the first piezoelectric thin film 80 was confirmed.The surface roughness of the first piezoelectric thin film 80 was about10 nm to about 50 nm according to the observation on the surface by anatomic force microscope (AFM).

Next, as shown in FIG. 7A, the first piezoelectric thin film 80 and thesecond interlayer film 60 are etched by using a photolithography methodso as to form the via hole 80 a to couple the CMOS circuit 30 to thesurface acoustic wave element 120, and then the Al electrode film 90 isformed with a thickness of about 1000 angstrom by using a sputteringmethod. The first piezoelectric thin film 80 (AlN film) is etched with astrong alkaline solution such as tetramethyl ammonium hydroxide (TMAH)while the second interlayer film 60 (TEOS film) is dry-etched. The Alelectrode film 90 is formed by using the reactive sputtering method inwhich the N₂ gas supply is stopped after being used for forming thefirst piezoelectric thin film 80 so as to form a pure Al electrode film.Then, as shown in FIG. 7B, the Al electrode film 90 is patterned byusing a photolithography method and etched so as to form theinterdigital electrode (the first electrode 90 a and the secondelectrode 90 b) for the surface acoustic wave element 120.

Next, as shown in FIG. 7C, the second piezoelectric thin film 100 (AlNfilm) is formed on the first electrode 90 a, the second electrode 90 b,and the first piezoelectric thin film 80 with a thickness of about 8000angstrom to about 15000 angstrom by using a reactive sputtering methodused for forming the first piezoelectric thin film 80. Since the secondpiezoelectric thin film 100 grows following the surface morphology ofthe first piezoelectric thin film 80, the second piezoelectric thin film100 grows to have a crystal grain size larger than that of the firstpiezoelectric thin film 80 (grown to about 500 nm). Accordingly, theridge-and-valley portion formed on the uppermost surface of the secondpiezoelectric thin film 100 is larger than that of the firstpiezoelectric thin film 80.

Subsequently, as shown in FIG. 8A, the sensitive film 110 on whichmolecules adsorb is deposited on the second piezoelectric thin film 100by using a CVD method or a sputtering method. Next, as shown in FIG. 8B,the sensitive film 110 is patterned by a photolithography method, andthen the sensitive film 110 is etched so that at least the film 100formed on the ridge-and-valley portion on the surface of the secondpiezoelectric thin film 100 is left. As a result, the sensitive film 110of the surface acoustic wave element 120 is formed.

Through the method described as above, the semiconductor sensor SS ofthe first embodiment can be manufactured. In addition, according to themethod of the second embodiment, the metal thin film 70 can be formed byusing conventional semiconductor manufacturing processes withoutemploying a particular process for enlarging the surface area of thesensitive film 110 as necessary in related art, e.g., JP-A-2007-147556.Thus, the manufacturing costs can be reduced.

The invention is not limited to the embodiments, the followingmodifications can be exemplified.

In the embodiments, the semiconductor sensor SS is exemplarily describedthat is provided with a single oscillation circuit including the CMOScircuit 30, the surface acoustic wave element 120, the feedback resistor200, the input side capacitor 210, and the output side capacitor 220.However, a plurality of oscillation circuits may be included. Forexample, in FIG. 9A, a semiconductor sensor SS' is shown that isprovided with 3 oscillation circuits. A first oscillation circuit (onlya CMOS circuit 30-1 and a surface acoustic wave element 120-1 are shownto be simplified for convenience) generates an oscillation frequency f1.A second oscillation circuit (only a CMOS circuit 30-2 and a surfaceacoustic wave element 120-2 are shown to be simplified for convenience)generates an oscillation frequency f2. A third oscillation circuit (onlya CMOS circuit 30-3 and a surface acoustic wave element 120-3 are shownto be simplified for convenience) generates an oscillation frequency f3.Measuring and averaging the changes of oscillation frequencies f1, f2,and f3 of the oscillation circuits allow more accurately detectingchemical substances.

For another modification, as shown in FIG. 9B, the surface acoustic waveelement 120-3 that does not have the sensitive film 110 is provided as areference element so as to use the oscillation frequency f3 as areference frequency. Comparing the oscillation frequencies f1 and f2 ofthe other oscillation circuits with the reference frequency allows moreeasily detecting the frequency varied by adsorption of gas molecules. Asa result, a semiconductor sensor having a higher sensitivity can beprovided.

In the embodiments, the CMOS inverter is exemplarily described as aninverter circuit used for the oscillation circuit. However,semiconductor elements other than CMOS may be used to structure theinverter circuit. In the embodiments, the surface acoustic wave element120 including two piezoelectric thin film layers is exemplarilydescribed, but more than two piezoelectric thin film layers may beincluded if required.

What is claimed is:
 1. A semiconductor sensor, comprising: asemiconductor substrate; a plurality of piezoelectric thin films layeredon the semiconductor substrate, the plurality of piezoelectric thinfilms including at least a pair of the piezoelectric thin films layeredabove and below; a pair of electrodes that are formed at an interface ofat least the pair of the piezoelectric thin films layered above andbelow and excite surface acoustic waves; a thin film directly under alowest-layer piezoelectric film of the piezoelectric thin films; a metalthin film that is formed at an interface of the lowest-layerpiezoelectric thin film and the thin film, having a ridge-and-valleyportion on a surface of an uppermost-layer piezoelectric thin film ofthe piezoelectric thin films; and a sensitive film for molecularadsorption formed on at least the ridge-and-valley portion on theuppermost-layer piezoelectric thin film, wherein a columnar crystalorientated and aligning c-axis direction of a wurtzite structure in theridge-and-valley portion.